<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>边框</title>
  <style>
    div {
      width: 100px;
      height: 100px;
      border: solid;
      border-radius: 20px;
    }
  </style>
</head>
<body>
  <div>这是一个div</div>
  <div class="box">
    <div id="cont">
        <div>111</div>
      
    </div>
  </div>
</body>
</html>